May I suggest using only some copper alloy, such as the brass brush you now have, to scrape off the rust? One may, or could, find bronze wool at grocery store or hardware, otherwise some piece of copper or brass can remove the rust without scratching that old iron.
For loosening rusted joints I find Kroil works better than, say WD-40.
Hey, Kroil is expensive, hard to find & has serious health warnings--Its gotta be good!